a) Field of the Invention
This invention relates to an improved electrowinning anode. The anode is a lead alloy sheet soldered to a copper busbar with a layer of lead electroplated over the busbar and the joint between the busbar and anode sheet.
b) State of the Art
Lead alloys have been used for many years as electrowinning anodes for recovery of copper, nickel and zinc from sulfate solutions. In most anodes the lead anode plate 1 is joined to the copper busbar 2 by casting lead 3 around the busbar as shown in FIG. 1. The lead cast around the busbar may be the same composition as the anode sheet, or a second alloy may be attached to the lead cast around the bar by welding or burning the sheet to the lead cast around the bar. The lead cast around the busbar serves as a barrier to attack of the busbar by acid mist produced in the electrowinning process. The lead covering also serves to conduct current from the copper busbar to the anode sheet.
A new method was developed to join a lead alloy sheet to a copper busbar in U.S. Pat. No. 4,373,654. In this system, shown in FIG. 2, a rolled lead-calcium-tin alloy sheet 4 is joined to the copper busbar 5 by means of a solder joint 6 in slot 7 of the busbar. This method gives a complete metallurgical bond between sheet and bar unlike that attained in the cast around system. In this system a thin coating 8 of lead tin alloy is formed on the surface of the copper busbar by dipping prior to the soldering process.
This thin coating produced by dipping in U.S. Pat. No. 4,373,654 is often not sufficient to completely protect the copper busbar from large amounts of acid mist or direct impingement of electrolyte onto the dip coated bar. Some attack of the copper bar can occur. The finished anode produced by this method cannot be coated with additional lead by dipping or casting around the bar because the lead or lead alloy used has a higher melting point than the solder in the joint, thus some remelting of the solder joint and loss of quality in the joint can occur.
The present invention provides a new method for completely protecting the copper busbar when producing anodes in which an anode sheet is soldered to a busbar by, for example, the method of U.S. Pat. No. 4,373,654.